Virtually all of the high speed elevators installed prior to 1975 used direct current (DC) motors. The source of the direct current was typically a motor-generator (MG) set. The alternating current (AC) motor of the MG set was connected to an AC supply grid powered by the three phase AC supply of the building.
Between 1975 and the early 1990's the majority of new high speed elevators were manufactured with DC motors supplied by a variable speed drive (VSD) that consisted of an isolation transformer, silicon controlled rectifiers, control electronics, and a ripple filter. This same VSD system was also used to modernize thousands of existing elevators. The existing DC motor was retained and the MG was replaced by the VSD.
The silicon controlled rectifier variable speed drive (SCR VSD) is considered to be much more energy efficient than the MG set because the MG set was turning even if the elevator was stopped. However, the SCR VSD wastes significant energy because the isolation transformer was always connected to the power supply grid. Additionally, the SCR VSD supplied standby power (approximately 50% of running current) to the motor field. This power was typically supplied 24 hours a day, 365 days a year. The only time the VSD was not connected and consuming power was during maintenance.